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2     <html>
3     <head>
4     <title>JSR 166 Snapshot Introduction.</title>
5     </head>
7     <body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000">
8     <h1>JSR 166 Snapshot Introduction.</h1>
10     by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a>
11     <p>
13     To join a mailing list discussing this JSR, go to:
14     <A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> .
16     <p>
17 dl 1.2 <em>
18     Disclaimer - This prototype is experimental code developed as part of
19     JSR166 and made available to the developer community for use
20     as-is. It is not a supported product. Use it at your own risk. The
21     specification, language and implementation are subject to change as a
22     result of your feedback. Because these features have not yet been
23     approved for addition to the Java language, there is no schedule for
24     their inclusion in a product.
25     </em>
27     <p>
28 jsr166 1.1 Package java.util.concurrent contains utility classes that are
29     commonly useful in concurrent programming. Like package java.util, it
30     includes a few small standardized extensible frameworks, as well as
31     some classes that provide useful functionality and are otherwise
32     tedious or difficult to implement. In this JSR, we have been
33     conservative in selecting only those APIs and implementations that are
34     useful enough to encourage nearly all concurrent programmers to use
35     routinely. JSR 166 also includes a few changes and additions in
36     packages outside of java.util.concurrent: java.lang, to address timing
37     and uncaught exceptions, and java.util to better integrate queues, and
38     to make Timers conform to new frameworks. The API covers:
40     <ul>
41     <li> Queues
42     <li> Executors
43     <li> Locks
44     <li> Condition variables
45     <li> Atomic variables
46     <li> Timing
47     <li> Barriers
48     <li> Concurrent Collections
49     <li> Uncaught Exception Handlers
50     </ul>
53     The main rationale for JSR 166 is that threading primitives, such as
54     synchronized blocks, Object.wait and Object.notify, are insufficient
55     for many programming tasks. Currently, developers can use only the
56     concurrency control constructs provided in the Java language
57     itself. These are too low level for some applications, and are
58     incomplete for others. As a result, application programmers are often
59     forced to implement their own concurrency facilities, resulting in
60     enormous duplication of effort creating facilities that are
61     notoriously hard to get right and even harder to optimize. Offering a
62     standard set of concurrency utilities will ease the task of writing a
63     wide variety of multithreaded applications and generally improve the
64     quality of the applications that use them.
66     <p>
67     Here are brief descriptions and rationales of the main components.
68     For details see the javadocs at <a
69     href="http://gee.cs.oswego.edu/dl/concurrent/index.html">http://gee.cs.oswego.edu/dl/concurrent/index.html</a>
72     <h2>Queues</h2>
74     A basic (nonblocking) Queue interface that is compatatible with
75     java.util.Collections will be introduced into java.util. Also,
76     although it is at the borders of being in scope of JSR-166,
77     java.util.LinkedList will be adapted to support Queue, and
78     a new non-thread-safe java.util.HeapPriorityQueue will be added.
80     <p>
81     Four implementations in java.util.concurrent support the extended
82     BlockingQueue interface, that defines blocking versions of put and
83     take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, and
84     PriorityBlockingQueue. Additionally, java.util.concurrent.LinkedQueue
85     supplies an efficient thread-safe non-blocking queue.
86     <p>
87     Since the target release is JDK1.5, and generics are slated to be in
88     1.5, Queues should be parametrized on element type. (Also some others
89     below.) We are ignoring this for now.
92     <h2>Executors</h2>
94     Executors provide a simple standardized interface for defining custom
95     thread-like subsystems, including thread pools, asynch-IO, and
96     lightweight task frameworks. Executors also standardize ways of
97     calling threads that compute functions returning results, via
98     Futures. This is supported in part by defining java.lang.Callable, the
99     argument/result analog of Runnable.
101     <p> While the Executor framework is intended to be extensible (so
102     includes for example, AbstractExecutor that simplifies construction of
103     new implementations), the most commonly used Executor will be
104     ThreadExecutor, which can be configured to act as all sorts of thread
105     pools, background threads, etc. The class is designed to be general
106     enough to suffice for the vast majority of usages, even sophisticated
107     ones, yet also includes methods and functionality that simplify
108     routine usage.
110     <p>
111     A few methods will also be added to the java.util.Timer to support
112     Futures, and address other requests for enhancement.
114     <h2>Locks</h2>
116     The Lock interface supports locking disciplines that differ in
117     semantics (reentrant, semaphore-based, etc), and that can be used in
118     non-block-structured contexts including hand-over-hand and lock
119     reordering algorithms. This flexibility comes at the price of more
120     awkward syntax. Implementations include Semaphore, ReentrantMutex
121     FIFOSemaphore, and CountDownLatch.
123     <p>
124     The Locks class additionally supports trylock-designs using builtin
125     locks without needing to use Lock classes. This requires adding new
126     capabilities to builtin locks inside JVMs.
128     <p>
129     A ReadWriteLock interface similarly defines locks that may be shared
130     among readers but are exclusive to writers. For this release, only a
131     single implementation, ReentrantReadWriteLock, is planned, since it
132     covers all standard usage contexts. But programmers may create their
133     own implementations to cover nonstandard requirements.
135     <h2>Conditions</h2>
137     A Condition class provides the kinds of condition variables associated
138     with monitors in other cocurrent languages, as well as pthreads
139     condvars. Their support reduces the need for tricky and/or
140     inefficient solutions to many classic concurrent problems. Conditions
141     also address the annoying problem that Object.wait(msecs) does not
142     return an indication of whether the wait timed out. This leads to
143     error-prone code. Since this method is in class Object, the problem is
144     basically unfixable.
145     <p>
146     To avoid compatibility problems, the names of Condition methods need
147     to be different than Object versions. The downside of this is that
148     people can make the mistake of calling cond.notify instead of
149     cond.signal. However, they will get IllegalMonitorState exceptions if
150     they do, so they can detect the error if they ever run the code.
151     <p>
152     The implementation requires VM magic to atomically suspend and release
153     lock. But it is unlikely to be very challenging for JVM providers,
154     since most layer Java monitors on top of posix condvars or similar
155     low-level functionality anyway.
157     <h2>Atomic variables</h2>
159     Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and
160     AtomicReference provide simple scalar variables supporting
161     compareAndSwap (CAS) and related atomic operations. These are
162     desparately needed by those performing low-level concurrent system
163     programming, but much less commonly useful in higher-level frameworks.
166     <h2>Timing</h2>
168     Java has always supported sub-millisecond versions of several native
169     time-out-based methods (such as Object.wait), but not methods to
170     actually perform timing in finer-grained units. We address this by
171     introducing java.lang.Clock, which provides multiple granularities for
172     both accessing time and performing time-out based operations.
175     <h2>Barriers</h2>
177     Barriers (multiway synchronization points) are very common in some
178     styles of parallel programming, yet tricky to get right. The two most
179     useful flavors (CyclicBarriers and Exchangers) don't have much of an
180     interface in common, and only have one standard implementation each,
181     so these are simply defined as public classes rather than interfaces
182     and implementations.
185     <h2>Concurrent Collections</h2>
187     There are no new interfaces, but JSR 166 will supply a few Collection
188     implementations designed for use in multithreaded contexts:
189     ConcurrentHashTable, CopyOnWriteArrayList, and CopyOnWriteArraySet.
191     <h2>Uncaught Exception Handlers</h2>
193     The java.lang.Thread class will be modified to allow per-thread
194     installation of handlers for uncaught exceptions. Ths optionally
195     disassociates these handlers from ThreadGroups, which has proven to be
196     too inflexible in many multithreaded programs. (Note that the combination
197     of features in JSR 166 make ThreadGroups even less likely to
198     be used in most programs. Perhaps they will eventually be deprecated.)
199     <p>
200     Additionally, Threads and ThreadLocals will now support a means to
201     clear and remove ThreadLocals, which is needed in some thread-pool and
202     worker-thread designs.
204     <hr>
205     <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>
206     </body>
207     </html>

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